The field of this disclosure relates generally to electric power systems, and more specifically, to the detection of defects in connecting rings for a doubly-fed induction generator (DFIG) rotor of a wind turbine.
Many known renewable energy facilities are coupled to an electric utility grid, and some of these facilities include wind turbines. Generally, a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. At least some known wind turbines are physically nested together in a common geographical region to form a renewable energy generation facility known as a wind turbine farm, sometimes referred to as a wind farm or a wind park.
Many known wind turbines include a DFIG that transforms mechanical energy into electrical energy. DFIGs include a multiphase rotor and a stator. The electrical phases of the multiphase rotor of the DFIG are coupled by a ring that is referred to as an end-ring, a neutral-ring, or a wye-ring. Some known DFIG generator rotors have a floating neutral point, which is frequently provided by the wye-ring. The wye-ring is typically made from a copper bar and is located at the non-drive end (NDE) of the generator. The wye-ring includes three connection points, i.e., one connection point for each phase of a three-phase rotor.
Due to operational stresses which fatigue the brazed connection between the wye-ring and its rotor connection points (or connection lugs), cracks can develop which lead to discontinuity. When a first crack occurs between two of the three connection points, the generator typically continues to function satisfactorily since current continues to reach all three rotor connection points. However, if a second crack, i.e., a two-crack condition, occurs in the wye-ring between another two connection points, at least one part, e.g., one phase, of the rotor windings is disconnected from the floating neutral in an open circuit condition, which can result in arcing across one of the cracks, and possible damage to insulation around the wye-ring and a resultant reduction in service life of the generator. The wind turbine monitoring system detects this cross-over arcing condition and recognizes it as a phase fault, and accordingly shuts the wind turbine down. Wye-ring failure can lead to an extended downtime and an expensive repair of the turbine.
A known method of testing for cracks in the wye-ring requires first taking the turbine offline, at least partially dismantling the generator, injecting a test current into connector lugs of the wye-ring structure, and then measuring the current flow, typically using an oscilloscope, at two or more of the wye-ring connection lugs. Taking the turbine offline creates an undesirable loss of energy production while the turbine is non-operational, and dismantling the generator before determining whether there is actually a crack in the wye-ring that requires repair can increase the maintenance cost of the turbine.